[Planning]: Convert existing "Executions" page to Cases or remove it

### Summary

_No response_

### Details

The executions search page was the original workflow for SimBoard before executions were encapsulated under cases. Cases are now the primary workflow for model runners, since that's how simulations are organized.

The executions page now serves as a cross-case comparison. However, there will be thousands of executions spanning different cases, which makes this page 1) hard to navigate and search 2) slow to apply filters (filters are really laggy right now).

### Deliverables

1. Assess the value and use cases for this page
2. Determine if case cross-comparison makes more sense through the case workflow

If the "Executions" page is determined to be obsolete, remove this page and all related code.

### Dependencies & Blockers

_No response_

### Due Date

_No response_

Start by reviewing the existing Executions search page and its filtering behavior, then document its current use cases and relationship to cases. Compare cross-case workflows and filter responsiveness; done means a recommendation to retain, move, or remove the page, with the related code scope identified if removal is chosen.

Written by the indexing model from the issue text.